Kieserite vs Heptahydrate: Choosing a Magnesium Source

The Question Behind Most Enquiries

Buyers regularly ask us to explain kieserite vs heptahydrate before placing a bulk order. Both are magnesium sulphate, but they differ in water content, magnesium concentration, solubility and how they behave inside a blending line.

Choosing between them is not a matter of quality. It is a matter of matching the form to the application, the equipment and the storage conditions.

Magnesium Sulphate Monohydrate Explained

magnesium sulphate monohydrate, commonly called kieserite, contains roughly 20% MgO with one molecule of water of crystallisation. It is dense, hard and comparatively low in moisture, which makes it a strong candidate for compound fertilizer granulation.

Because it carries less water per unit of magnesium, it adds less dead weight to a blend and reduces the moisture load in storage.

Magnesium Sulfate Heptahydrate Explained

magnesium sulfate heptahydrate, or epsom salt, carries seven water molecules and about 9.8% MgO. It dissolves quickly, which is why it dominates foliar sprays, fertigation and any application where a clear solution is required.

Its higher moisture content means more care in humid storage, but the fast dissolution is a genuine process advantage in liquid systems.

kieserite vs heptahydrate — Magnesium Sulphate Monohydrate(Kieserite) granular W.MgO20%23%25%min from MgSO4 Group Limited — view product: https://www.mgso4.com/pid18135930/Magnesium-Sulphate-Monohydrate-Kieserite-granular-W-MgO20-23-25-min.htm

Comparing MgO Content

MgO content is the fairest way to compare cost, because a per-tonne price ignores how much magnesium each form actually delivers. Roughly speaking, a monohydrate grade supplies about twice the MgO of a heptahydrate grade by weight.

Recalculate your quotation per kilogram of delivered MgO before deciding. A higher bag price can still be the cheaper nutrient source.

Behaviour in Magnesium Sulfate Fertilizer Blends

In a dry magnesium sulfate fertilizer blend, the harder kieserite granule improves granule strength and generates fewer fines. Heptahydrate crystal works well in a broadcast application but can soften and cake if the blend is stored through a humid season.

Where the blend includes hygroscopic nitrogen components, the lower moisture contribution of monohydrate is usually decisive.

Choosing a Granular Magnesium Source

A granular magnesium source should match the particle-size distribution of the other blend components to limit segregation during transport. Ask for the sieve band on the COA rather than a generic granular description.

Run a short segregation trial in your own mixer, then sample the blend at several discharge points to confirm uniformity.

Solubility and Application Method

If the programme is dissolved for fertigation or foliar use, heptahydrate is the straightforward choice. Monohydrate dissolves more slowly and is normally selected for soil or blend application instead.

Do not assume a product labelled water soluble behaves identically at low water temperature; validate dissolution time under your own conditions.

Storage and Shelf Considerations

Both grades should be kept sealed, palletised and off concrete floors. Heptahydrate needs tighter humidity control, while monohydrate tolerates a wider range but still benefits from dry, cool storage.

Rotate on a first-in basis and re-check flow behaviour after a long sea voyage, especially for tropical destinations.

magnesium sulphate monohydrate — Magnesium Sulphate Heptahydrate (Epsom Salt) 99.5% 1-3 mm crystal

A Simple Decision Rule

Choose kieserite for granulation, bulk blending and long storage where MgO density matters. Choose heptahydrate for dissolution, foliar work and rapid correction. Many programmes use both, at different points in the season.

MgSO4 Group Limited supplies both forms with declared size bands and per-lot COAs. Share your process and we will recommend a starting grade and sample.

Share your grade, quantity and destination with MgSO4 Group Limited for a specification-matched quotation and documented lead time.